In virtually every radiotelephone system, a large number of subscribers are served from a central fixed site which provides communication channels to the subscribers as the channels are needed and as the subscribers move from place to place. Of course in any such system it is likely that the number of subscribers will occasionally exceed the number of communication channels available. This problem has been addressed in many ways, including providing a busy signal to the subscriber (that is, simply not providing service) and constructing cellular systems having the capacity to reduce the sizes of the radio coverage areas (cells) and to subsequently place more cells and hence more communications channels in a geographic area. Needless to say, not providing service is an unacceptable solution while the process of cell splitting and redeployment is slow and not responsive to a dynamic radiotelephone system.
Digital radiotelephone service has been proposed as another method of increasing the number of channels available to subscribers. Two of the most promising digital methods, time division (TD) and code division (CD), offer increased channel availability by further sharing the physical resources among the subscribers. Simply stated, analog signals (such as voice) are converted (in a vocoder) to a digital signal using a conversion format having specified sampling frequency and a particular data compression technique. The compressed data is combined with the time division or code division signal and transmitted, via radio, to the receiver. The receiver demodulates the time division or code division transmitted signal and decompresses and resamples the data in a process complementary to the conversion prior to transmission. This produces a reproduced analog signal at the receiver.
The radiotelephone service is bi-directional in that the fixed station both transmits and receives vocoded signals and the subscriber unit reciprocally transmits and receives vocoded signals. Compatibility between transmitting units and receiving units requires that the conversion rates and compression techniques be opposite and equal. Whatever rate is used in the analog to digital processing must be matched in the corresponding digital to analog processing. It should be noted that, in general, the lower the vocoder bit rate, the greater the number of available communications channels, but the quality of the speech is reduced.
It has been suggested that the vocoding rates be made variable to encode speech signals at rates based upon the characteristics of the speech to be encoded. In order that the optimum quality of speech be transmitted, the highest rate is used during passages of speech. When the speaker pauses or stops speaking, a variable rate vocoder reduces the rate, thus lowering the average rate per talker, which allows the system to provide more communications channels. See, for example, U.S. Pat. No. 5,515,375 "Method and Apparatus for Multiplexing Fixed Length Message Data and Variably Coded Speech" filed on behalf of DeClerck on Jul. 30, 1993 and assigned to the assignee of the present invention.
Even employing the foregoing techniques may not yield a radiotelephone system offering a sufficient number of communication channels in an efficient manner to accommodate a large subscriber population.